<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .header {
            width: 100%;
            height: 200px;
            background-color: red;
            position: fixed;
            top: 0;
        }
        .box {
            width: 100%;
            height: 100%;
            position: fixed;
            display: flex;
            margin-top: 250px;
        }
        .box .left {
            overflow: auto;
            width: 200px;
            /* height: 1000px; */
            background-color: pink;
        }
        .box .right {
            overflow: auto;
            flex: 1;
            /* height: 1000px; */
            background-color: skyblue;
        }
        .box  ul li {
            width: 100%;
            height: 400px;
            margin-top: 50px;
            background-color: black;
        }
    </style>
</head>
<body>
    <div class="header"></div>
    <div class="box">
        <div class="left">
            <ul>
                <li></li>
                <li></li>
                <li></li>
                <li></li>
                <li></li>
            </ul>
        </div>
        <div class="right">
            <ul>
                <li></li>
                <li></li>
                <li></li>
                <li></li>
                <li></li>
            </ul>
        </div>
    </div>
    <script>
    </script>
</body>
</html>